Cooling issues

My M1123 has been giving me a few issues with the cooling system as of late and I'm having a hard time tracing the source.

Pertinents- M1123- 6.5 GEP 506 - New TDM (April 2022) and Thermostatic switch (August 2022) from Midwest Military Equipment, Cadillac valve tests at 78 Ohms, Power Steering fluid level checks out, fan engages and disengages when shorted.


The HMMWV has been running fine all season, rests consistently at 205 per the gauge, almost zero fluctuation there. About a week ago, I open the hood to check fluids and get ready for an oil change (500 miles on Odometer) and see that the washer fluid shelf has some red fluid on it, not enough to drip onto the floor, but enough to see it saturating the paint. I cannot say for certain, but I believe it was the return line to Power Steering from the Cadillac valve. I check fluid level in Power steering and sure enough, it's low, not empty, but low. It takes a few days for the Dexron III to show up, I add about 3 ounces and it's sitting pretty at the cold fill mark. No errant noises from Power steering or otherwise. I cut the Power steering return hose back about 1.5" and re-seat the hose clamp, no leaks or otherwise since.

Here's where it gets tricky, now the truck is reading an ever creeping temperature until it registered about 235 on the gauge and I shut it down. I begin to trouble shoot, test the Cadillac valve - it reads good at 78 OHMs. The fan is disengaged when started from cold and everything is plugged in, but turns on when electrical is unhooked, so I'm thinking the Cadillac valve is ok. When the Thermostatic switch is unhooked and that circuit is jumped, the fan kicks on, so I'm thinking the TDM is ok. This leads me to the Thermostatic switch, which I figured was faulty, and may have been causing the heating issue. I ordered a new one and received and installed it this morning. After a quick 10 mile run, it's doing the same thing, slow creep up in the temperature range until I kill the engine.

This is basically where I am with it at this point. I will add that the temperature at the Thermostatic switch reads about 180 degrees with an infrared thermometer gun, so there is clearly a discrepancy between the switch and the temperature sensor. My question then becomes, how should I proceed? I am hesitant to let the temp keep rising, for fear that the coolant temp just isn't registering with the infrared thermometer from the outside of the engine. I also can't run the truck like this, as the temperature gauge would basically be pegged all the time and no longer give indication, should something genuinely be amiss.

So throw it at me, what should be my next step in diagnosing whatever the heck is going on here.

The switch is a fail safe design, it should engage the clutch when disconnected as in normally closed when cold.
At 235 degrees you should be getting some indication it is running hot.
Does the fan kick on? if not it is ether too cold or there is another problem, you could wait until it registers hot as in 220 or higher and pull the cad valve plug and see if it cools down, if it is the gauge I would think it would show it was cooling off more than normal.

Mogman said:

The switch is a fail safe design, it should engage the clutch when disconnected as in normally closed when cold

Click to expand...

Right, and it does when disconnected, but when the truck is used normally, it does not come on, despite temperatures appearing to exceed 235 degrees per the famously inaccurate gauge. The question I have is how do I determine what needs to be replaced or adjusted without risking overheating my engine.

Per the edit- No indication it's running hotter than normal, but I am still pretty new to this truck and HMMWVs in general, so I don't know what to be looking for.

The fan does not kick on unless bypassed by shorting the thermostatic switch or unplugging the Cadillac valve.

Good call on unplugging it and watching what happens, I'll go give that a shot and report back!

Sometimes when an engine is actually overheating it can "thump" after being shut off due to the water boiling in areas inside the engine that are hotter then the gauge is registering.
(may be thumping while running but you will probably not hear it).
And things will tend to be smoking on the engine because oil residue etc. is getting hotter than usual so it starts to cook off.
And of course the coolant reservoir may have a higher than normal level due to the water expanding more than normal.
BUT you said.

"The fan does not kick on unless bypassed by shorting the thermostatic switch or unplugging the Cadillac valve."

The fan should kick on when the switch is disconnected not shorted.

Mogman said:

Sometimes when an engine is actually overheating it can "thump" after being shut off due to the water boiling in areas inside the engine that are hotter then the gauge is registering.
(may be thumping while running but you will probably not hear it).
And things will tend to be smoking on the engine because oil residue etc. is getting hotter than usual so it starts to cook off.
And of course the coolant reservoir may have a higher than normal level due to the water expanding more than normal.
BUT you said.

"The fan does not kick on unless bypassed by shorting the thermostatic switch or unplugging the Cadillac valve."

The fan should kick on when the switch is disconnected not shorted.

Click to expand...

So I got that backwards, when the thermostatic switch is unplugged it turns on, when it's shorted it turns off, I had that backwards - you are correct.


I just took it for a little drive and the temp began to rise, then I pulled over and unplugged the Cadillac valve, and she was resting at the same 205 degrees (indicated on gauge, not reality) as she always does. This leads me to believe that I've just been driving it with the fan on for the past 200 miles or so.

So I would guess that my issue here is that I have a Temperature gauge that's reading WAY off of reality. I'm going to take the old Thermostatic switch and try it in some boiling water, see if I can get different reads from it. If it reads healthy, I'd guess it's the Temperature sensor on the side of the block, does that logic compute?

If it does make sense that it's just reading very high and I may have had the Cadillac valve inadvertently unplugged all this while, how would I go about getting the Temp gauge to read more accurately?


To clarify on how the Cadillac valve could have been disconnected for 200 miles - the TDM I bought from Midwest, is very clearly a knockoff - as is the replacement Thermostatic Switch, I'm pretty pissed off about that, to be honest. Both pictures on their website show a very different item than I received. It wasn't until the leak happened that I looked more closely at the switch and saw that one of the pins was not seated properly. At that point I was chasing which problem occurred first.

SmartDrug said:

So I got that backwards, when the thermostatic switch is unplugged it turns on, when it's shorted it turns off, I had that backwards - you are correct.


I just took it for a little drive and the temp began to rise, then I pulled over and unplugged the Cadillac valve, and she was resting at the same 205 degrees (indicated on gauge, not reality) as she always does. This leads me to believe that I've just been driving it with the fan on for the past 200 miles or so.

So I would guess that my issue here is that I have a Temperature gauge that's reading WAY off of reality. I'm going to take the old Thermostatic switch and try it in some boiling water, see if I can get different reads from it. If it reads healthy, I'd guess it's the Temperature sensor on the side of the block, does that logic compute?

If it does make sense that it's just reading very high and I may have had the Cadillac valve inadvertently unplugged all this while, how would I go about getting the Temp gauge to read more accurately?


To clarify on how the Cadillac valve could have been disconnected for 200 miles - the TDM I bought from Midwest, is very clearly a knockoff - as is the replacement Thermostatic Switch, I'm pretty pissed off about that, to be honest. Both pictures on their website show a very different item than I received. It wasn't until the leak happened that I looked more closely at the switch and saw that one of the pins was not seated properly. At that point I was chasing which problem occurred first.

Click to expand...

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These gauges are notoriously bad. More used to see what if it shows as normal today, then see it again in the same place tomorrow. AND if it suddenly is different one day - that might be a problem. A new gauge or sending unit or a cleaned up ground might make everything better.

Sounds like you are already on the way to solving the rest of the problem...

Camo might jump in here and list the ohms/temp of the sending unit, yes IIRC it is on the side of the block, again IIRC under the alt where it is a PITA to get to.
You may want to re-install the original thermostatic switch, if when you disconnect it the fan comes on then the TDM and the cad valve is not the problem but the knock off temp switch might be a problem.
As Mullaney said you might try cleaning the inst. ground behind the inst panel and the other side where IIRC the chassis harness ground connects. but usually a bad ground causes erratic behavior.
ALSO you never said if the fan was NOW coming on around 200 or so, if not then that could be the problem,,, after discovering the connection problem keeping the fan on all the time and fixing that the fan may not be coming on at all now, granted the thermostat is a 190 deg but it may not go fully open until around or 200 so which would make the gauge not that far off and if the fan was running all the time the thermostat would be controlling the temp.

So the gauge is reading about 205 with the cadillac valve unplugged.
To me that tells me that either the thermostat isn't opening at the proper temp, or that my gauge is off.
With the fan fully engaged you should be seeing 180-190 degrees. You are reading about 20 degrees above that.
You also said you used the laser thermometer on the block next to the thermostatic switch and it is about 180 when the gauge is reading much higher.

Use your laser thermometer and measure next to the temp sending unit, as well as the thermostat housing and the hoses.
If the thermometer isn't agreeing with what your gauge reads at those locations, then it's the gauge or sending unit that are off.

As you know, the gauges are notoriously inaccurate. You seem to be chasing around an overheating problem and replacing parts for the cooling system because your gauge is reading high, but haven't actually verified the gauge.

I made a grounding harness when I got her, but I did run into some odd oil pressure reads a few weeks ago that was fixed by checking ground and some dialectic grease, so I’ll go do some cleaning of the gauge grounds, good call. I don’t know why I didn’t think of that.

Also, good call on the thermostat basically controlling the temperature when the fan is always on. That would mean it was reading about 15 degrees high, assuming it opened at 190 and read around 205. Given that, maybe I just need to lock the throttle, pop the hood, and watch temps at the thermostat with the IR thermometer- on that note, do you know how much discrepancy you see from the coolant to the thermostat housing? What my upper temp limit should be?

Coug, good call, I’ll check on the block, too.

Thermostats are rated on the temperature at which they begin to open and allow coolant to flow through. This opening temperature is usually 195 degrees Fahrenheit on cars. As the engine heats up, the thermostat opens more to allow freer coolant flow. At about 20 degrees above its rated temperature, the thermostat is fully open.

Do you think I’m safe to let it run until the thermostat housing is at 220 degrees or so?

I’m guessing that it’ll read somewhat lower than the actual coolant, but don’t want to risk friction welding my pistons to the block just to confirm that the fan does actually engage properly.

Mogman said:

Camo might jump in here and list the ohms/temp of the sending unit, yes IIRC it is on the side of the block, again IIRC under the alt where it is a PITA to get to.
You may want to re-install the original thermostatic switch, if when you disconnect it the fan comes on then the TDM and the cad valve is not the problem but the knock off temp switch might be a problem.
As Mullaney said you might try cleaning the inst. ground behind the inst panel and the other side where IIRC the chassis harness ground connects. but usually a bad ground causes erratic behavior.
ALSO you never said if the fan was NOW coming on around 200 or so, if not then that could be the problem,,, after discovering the connection problem keeping the fan on all the time and fixing that the fan may not be coming on at all now, granted the thermostat is a 190 deg but it may not go fully open until around or 200 so which would make the gauge not that far off and if the fan was running all the time the thermostat would be controlling the temp.

Click to expand...

CAMO on the hi dive into hot water... The temperature guage to test tracking works out to...

OPEN circuit = no reading
Short circuit = full scale
120 degrees =2250 ohms
160 " = 1700 ohms
200 " = 890 ohms
240 " = 640 ohms

This data is useful to check the guage in place with sender removed. I have found the wonky reads are to mechanical failures due to water intrusion. (rusted, busted, gummed up or just plain worn out. Good batch of guages average +/- a needle width with the test above..

NOW it is important that grounds and don't get all crazy on me with grounding stories... BUT the little horse shoe brackets and loose nuts (that funny) to make the ground to the meter stud are most problematic. Our fix is to " double nut the stud to the lug". Have seem many gauges held in and grounded with the same one nut. Over time bracket bend, never tight enough, plastic remolds, corrosion sets in... loss of connection and there goes any chance of a reliable reading. Outside temperature, moisture, time of day or just bang on it like a Las Vegas slot machine till it plays nice.

And to the electrical fan control... That thermostatic switch in the crossover is the heart of it all. YES, it work backwards conventional wizdum. Switch On is fan OFF and OFF is fan ON. Pulling any plug, system goes into fail safe and that's FAN ON.

Fan ON and... MY guess, it would be hard to get to 190 degrees unless puling a heavy load. IMO... with everything else clean and pumping, at pressure, and correct water thermostat. Note those switches go bad in all ways possible. Buy quality "BLUE TAG" do not china out on this item.

When fan is engaged...It's a hurricane, CAMO at Camoteksystems.com

I do not think you should let it run hot even if it is only a faulty gauge, you need to get a handle on what is going on, you still have not said if the fan is running at all now that you have "fixed" the poorly connected connector or not, any time you drive in the summer it is NORMAL for the fan to cycle, if not you have a PROBLEM unless you have the best damn radiator and air flow of any HMMWV that I have seen, and if it does it should reduce the temp, maybe not to the 205 as was indicated when the fan was running all the time.

I would be weary of a 235 degree indicated.

If you cannot hear the fan cycle then put a volt meter on the cad valve wire and run it into the cab and see if the 24V cycles off or not.

EDIT, I see Camo has arrived!

What I would do is heat it up to 160 indicated, shut down and pop the wire off of the sending unit and ohm it to ground, if it reads around 1700 ohms then the GAUGE is reading right, but it could be a bad sending unit, so pop the sending unit out put it in your wife's best pot and bring it to a boil and see if it reads around 800 ohms or so.

Actually it would be best to pop the wire off while it is cold and use a jumper (alligator) lead so when you have 160 indicated you do not burn the crud out of yourself disconnecting the wire.

Thank you all for the assistance here. It is appreciated.

The fan is not coming on from my driving, but I’m also not seeing thermostat housing temps above 190 degrees. This truck has seen very light duty thus far- driving to Home Depot, taking the kids to the pool and on hikes, etc.- the longest drive has been about 30 miles at 30-45 MPH- the joys of suburban Chicago. I’m still testing it out, making sure I’ve buttoned up all the loose ends. Bear in mind that all that driving has likely been with the fan on full time. It wasn’t really until the last 20 miles - also around town- that the fan is off, hence the rising temperature and the ques

The other thing at work here is that almost all my driving has been done with aviation headsets on so I could talk with the family. It has since dawned on me that this is almost certainly why I didn’t realize that the bloody fan has been in the whole time. It did sound like a hurricane driving around, but I was 19 and stupid the last time someone let me check one out of the motor pool, so I didn’t realize anything was amiss- Idiot HMMWV newbie mistake. Coolant held nicely in the safe zone, truck drove fine, albeit slow, MPGs were low, but it’s all city driving, nothing stuck out as off to me.

Gauges are all double nuts, ground inbetween them. As of this evening, all have been cleaned up and diaelectric greased. I’ll be doing some checking of some sending unit Ohms tomorrow, in addition to reinstalling the old thermostatic switch, maybe that will yield some fruit.

Update for today -

Seeing 160 on the gauge, I'm seeing 1400-1500 Ohms at the sensor. Simultaneously I’m seeing the temp on the thermostat housing of 130 degrees and the temp gauge sensor of 130 as well. The block itself is reading about 160, FWIW.

Based on that, I’m thinking that, while the gauge is still somewhat innacurate, the issue is a temperature sensor that is giving bad data.

What does the hive mind think?

1500 is not that far off of 1700, I would suspect the sender, If it were me I would put the sender in a pot and test it, but that is cuz I am retired and it is too dang hot to hand around outside.

If your seeing 235, keep going…I’ve seen manny truck not engage the fan till 235ish “indicated“
I’ve also seen many times a faulty Cadillac valve solenoid…in your case, your getting voltage which leaves the fan clutch
disengaged, but never denying voltage, that denial only comes from your thermostatic switch.
I’ve also had my share of temp gauges that were way off…

Retiredwarhorses said:

If your seeing 235, keep going…I’ve seen manny truck not engage the fan till 235ish “indicated“
I’ve also seen many times a faulty Cadillac valve solenoid…in your case, your getting voltage which leaves the fan clutch
disengaged, but never denying voltage, that denial only comes from your thermostatic switch.
I’ve also had my share of temp gauges that were way off…

Click to expand...

RWH- that was my mentality as well. If the other components function, it’s just down to the mechanical switch to complete the circuit at the appropriate temperature.
That said, I will still need to get the gauge sorted before I’ll trust driving this around. Otherwise it’ll be pinned at full range almost all the time, my concern is that it would provide almost zero warning if anything fails. Given that, I’m going to try flushing the system again and replacing the sensor, and possibly the gauge as well, hopefully I get somewhere tomorrow when the sensor shows up and I can flush the coolant again.